There is an increasing demand for a small, lightweight, highly flexible actuator in the fields of medical instruments, industrial and personal robots and micromachines, and the like.
When actuators are produced in small sizes, it is difficult to use motors, engines and like mechanisms that convert energy into movement by means of inertial force as power sources for such small actuators because friction and viscous forces become dominant relative to inertial forces. Therefore, electrostatic attraction, piezoelectricity, ultrasonic waves, shape memory alloys, and polymer expansion/contraction have been proposed as operational principles for small actuators.
Such small actuators, however, are problematic in that, for example, their operating environments are limited, responsivity is insufficient, structure is complex, and flexibility is wanting. The applicability of such actuators is accordingly limited.
In order to solve such problems and broaden the application of small actuators, polymer actuators have been developed that can be driven at low voltages, exhibit a prompt response, have high flexibility, can be easily made in small sizes and weight-reduced, and operate with little electricity. Such polymer actuators are roughly divided into two types, i.e., those utilizing expansion/contraction due to reduction/oxidation in an electrolyte of an electron-conductive polymer such as polypyrrole or polyaniline (electron-conductive polymer actuators), and those composed of ion-exchange membranes and electrodes disposed thereon that can function as actuators due to the flexion and deformation of ion-exchange membranes that occur in response to the application of a potential difference to the ion-exchange membranes that are in a state of containing water (ion-conductive polymer actuators).
Among such actuators, electron-conductive polymer actuators are advantageous in being able to be driven at low voltages, expand/contract to a great extent, and generate high pressure. However, their response is slow, and the only method for producing polypyrrole, which is most advantageous, is electrolytic polymerization. Moreover, it has been pointed out that their durability for repetitive use is theoretically questionable because the response is due to the doping and undoping of ions based on the redox reaction.
To overcome such problems, an actuator has been proposed in which electrodes taking the form of a paper prepared from carbon nanotubes undergo expansion/contraction due to the interfacial stress change resulting from double-layer charging/discharging (see non-patent literature 1). This actuator should have a fast response and a long lifetime considering the principle of double-layer charging/discharging. Moreover, the pressure generated by this actuator has been proved to be large. However, the extent of expansion/contraction is small, and the production method involves filtration over a long period of time, a very complex operation. In addition, this actuator has a low mechanical strength and operates only in electrolyte solutions.
Prior-art electron-conductive polymer actuators and ion-conductive polymer actuators require electrolytes for their operation, and are thus mostly used in aqueous electrolyte solutions. Ion-conductive polymer actuators do not exhibit sufficient ion conductivities unless ion-exchange resins are in the condition of being swelled by water, and thus are usually used in water. Using such actuators in air requires water evaporation to be prevented. A resin coating method has been proposed for this purpose. However, this method is not currently in practice because complete coating is difficult, the coating gets broken by even a small amount of gas generated by electrode reaction, and the coating itself serves as a resistance to response deformation. Although high-boiling-point organic solvents such as propylene carbonate can be used in place of water, such solvents pose similar problems, and are further problematic in having neither so much ion conductivity nor responsivity as water.
Hence, prior-art actuators mainly function only in restricted environments, i.e., in electrolyte solutions, and therefore, their application is very limited. Development of an actuator that can operate in air is essential for using small actuators in a broad range of applications.
There are some examples of actuators operating in air, in which an electron-conductive polymer is disposed on both sides of an ion-exchange resin, or a conductive polymer is disposed on a gel membrane containing a high-boiling-point organic solvent such as propylene carbonate, and these examples utilize the expansion and contraction of electrodes disposed on both sides of the substrate for use as actuator elements. These examples, as with ion-conductive polymer actuators, do not overcome the problem of solvent evaporation and low ion conductivity, and do not serve as fundamental solutions.
To solve such problems, applied research has recently been carried out using salts that are in a molten state within a broad temperature range including ordinary temperatures (room temperature), known as ionic liquids and called room-temperature molten salts or simply molten salts. Ionic liquids have negligible vapor pressure and are thus free from solvent evaporation.
For an electron-conductive polymer actuator to operate in air, research has been carried out on the expansion/contraction of conductive polymers in ionic liquids (non-patent literature 2), and on entirely solid-state elements using a complex of polypyrrole, an ionic liquid, and polyvinylidene fluoride (non-patent literature 3). However, the aforementioned fundamental problems resulting from conductive polymers, i.e., slow responsivity, production method, and lifetime, have not yet been solved by such research.    Non-patent literature 1: Science, Vol. 284, 1999, p. 1340    Non-patent literature 2: Science, Vol. 297, 2002, p. 983    Non-patent literature 3: Electrochimica Acta, Vol. 48, 2003, p. 2355